At a rally for President Obama in September 2012. As the first black president, Mr. Obama stirred an unprecedented level of enthusiasm and turnout among black voters.CreditCreditDamon Winter/The New York Times

So here’s a simple question: Why haven’t these demographics swept Hillary Clinton to a big polling lead and a smooth glide to victory? Donald J. Trump, after all, has alienated just about every growing demographic group and every category that helped push Mr. Obama to victory.

The biggest reason is that demographic change was an overrated contribution to Mr. Obama’s victory, and it will help Mrs. Clinton only at the margins this year. Analysts have conflated all of the effect of higher turnout and percentage of support among nonwhite voters with demographic shifts. In truth, the turnout and support were far more powerful components.

Mrs. Clinton is not poised to match the gains Mr. Obama made among nonwhite voters over previous Democratic nominees. That brings the pace of Democratic gains down to the slow crawl of demographic change.

Demographic Change Not as Powerful as Assumed

The traditional demographic story is fairly simple: Between 2000 and 2012, the white, non-Hispanic share of voters plummeted. According to the census, white, non-Hispanic voters represented just 74 percent of the electorate in 2012 — down from 81 percent in 2000.

The shift was, indeed, driven by demographic change. The white, non-Hispanic share of adult citizens — roughly the pool of people eligible to vote — fell by roughly the same amount over the same period.

The white, non-Hispanic share of adult citizens who are eligible voters has continued to fall — probably down to around 68 percent, although the census information is not yet up to date.

The growing diversity of the electorate has undoubtedly helped the Democrats. However, many people look at these numbers and assume that the preponderance of Democratic gains over the last decade can be attributed to these shifts. That is not so.

In fact, John Kerry would have probably lost the 2004 election even if eligible voters had been just as diverse as they were in 2012.

Conversely, Mr. Obama would have probably won his two elections even if the last decade of demographic shifts had never happened.

How? Because Democratic gains come from three largely distinct sources: demographics, support and turnout.

Demographic trends did help Democrat nominees, a little. But they were helped just as much by favorable turnout trends — the huge surge in black turnout, in particular — and by favorable trends in support. Mr. Obama did much better than Mr. Kerry among Hispanic voters, among black voters and even among white voters outside the South and Appalachia.

On balance, these other factors — support and turnout — not only outweighed demographic shifts as drivers of growing Democratic strength, but they also made demographic shifts more powerful. Demographic change, for instance, wouldn’t have done Mr. Kerry nearly as much good as it did Mr. Obama because Mr. Kerry didn’t do as well among Hispanic and Asian voters — the main drivers of demographic shifts.

Here’s one way to look at it: Between 2004 and 2012, the Democratic margin among Hispanic voters roughly doubled, from 18 percentage points to 38 points, according to The Upshot’s estimates, which are based on pre-election polls, census data and the actual results.

But over the same period, the number of Hispanic voters increased by just 20 percent, a far smaller contribution to Democratic strength.

The impact of turnout and support over demographic shifts was even greater in the battleground states. Many of the key Northern battleground states — Pennsylvania, Ohio, Iowa, New Hampshire and Wisconsin — saw well-below-average changes in the composition of the electorate. In these states, Mr. Obama nonetheless ran ahead of Mr. Kerry by energizing black voters and doing better among white voters as well. Even the states with well-known “demographic shifts” — like North Carolina and Colorado — actually had below-average demographic shifts.

And many of the biggest demographic shifts occurred in the Sun Belt states where Democrats talked about being competitive, like Georgia and Arizona, even Texas, but really weren’t. Demographic shifts in these areas were often canceled out by considerable losses among white voters over the same period.

My view is that this is somewhat coincidental. Many of the red states with growing nonwhite populations also have the sort of older, Southern, less educated and more religious white populations that have trended toward the Republicans, even in states with far less demographic change and far fewer nonwhite voters (Arkansas, Oklahoma, Kentucky, Missouri). The states with rapid demographic change but fewer Southern white voters — like California or New Jersey — did not see the same weakening in Democratic strength.

Hispanic Support Appears Flat

Yes, demographic shifts will continue to slowly help Democrats. But Mrs. Clinton isn’t getting the same leaps in support and turnout among nonwhite voters that let Mr. Obama grow the Democratic coalition as much as he did.

On average, Mrs. Clinton leads among Hispanic voters by almost the exact same amount that Mr. Obama did in pre-election polls in 2012.

This is lost in many articles that cherry-pick the most shockingly pro-Clinton results. The results that don’t show her doing so well — like a Pew poll showing her leading by 50 to 26 — are dismissed, even though the same pollsters four years ago showed Mr. Obama faring about as well as he ultimately did.

Another reason Mrs. Clinton’s relative weakness among nonwhite voters has been overlooked is that analysts and journalists have tended to focus on how Mr. Trump is doing worse than Mr. Romney (Mr. Trump has only 15 percent support among Hispanics compared with Mr. Romney's 27 percent in the exit polls). But they leave out that Mrs. Clinton, by the same measure, is doing worse than Mr. Obama to the same extent.

A final problem is that a handful of polls specifically targeting Latino voters tend to show Mrs. Clinton ahead by a larger margin than other surveys do. These pollsters argue that they’re a more accurate reflection of the Latino electorate. Whether this is true is beside the point: The methodology employed by these surveys has always yielded stronger results for Democrats than other surveys, including the exit polls.

As a result, comparing these polls with the exit polls tends to show Democrats gaining when, in fact, they may not be at all. Mr. Obama, for instance, led the final Latino Decisions poll by a margin of 75 percent to 23 percent, about the same as Mrs. Clinton's current 71-19 lead in a recent survey.

Why isn’t Mrs. Clinton doing better than Mr. Obama among Hispanic voters? Part of her problem, I suspect, is that Hispanic voters are disproportionately young, and she has had a tough time rallying the support of young voters. It is also possible that she is struggling among less educated second- or third-generation Hispanic voters who may vote more similarly to less educated white voters.

Mrs. Clinton isn’t doing better than Mr. Obama among black voters, either. While several polls have suggested that Mr. Trump is winning a vanishingly small share of that vote, the polls showed something similar in 2012. Then, pre-election polls showed Mr. Obama beating Mr. Romney by an even greater margin than the polls currently show Mrs. Clinton beating Mr. Trump. Mr. Obama held a 93-3 percent lead among likely black voters (92 to 3 among registered voters).

Most of the analysis that shows Mrs. Clinton faring better depends on a comparison with the exit polls, which showed Mr. Romney winning 6 percent of the black vote.

My view is that the pre-election polls were probably right, and that Mr. Obama won black voters by an even greater margin than the exit polls found. The exit polls are not good at measuring clustered groups, as the exit pollsters have long acknowledged. But regardless of which measure most accurately depicts the electorate, the apples-to-apples comparison is the sounder one. It shows more stability than change.

Mrs. Clinton is also in danger of losing at least some of the favorable shifts in turnout between 2004 and 2012, which benefited Mr. Obama. Black turnout may not exceed white turnout again, either because black turnout fades a bit, or because white turnout increases. I’m agnostic on this question: It is very difficult to project turnout with that kind of precision. But there’s more room for Mrs. Clinton to fall here than to make additional gains.

The bottom line is that Mrs. Clinton is unlikely to benefit from the same jump in black turnout and support that Mr. Obama had. Similarly, she is unlikely to repeat the same jump in support from Hispanic voters. It is possible she won’t see any gains among these groups at all.

Democratic gains from nonwhite voters have greatly slowed, to the marginal gains from demographic shifts alone.

The end result: According to The Upshot’s estimates, Mrs. Clinton’s gains from demographic change could be as small as a single percentage point.